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  • Traveling reference spectroradiometer for routinequality assurance of spectral solar ultravioletirradiance measurements

    Julian Grbner, Josef Schreder, Stelios Kazadzis, Alkiviadis F. Bais, Mario Blumthaler, Peter Grts,Rick Tax, Tapani Koskela, Gunther Seckmeyer, Ann R. Webb, and Diana Rembges

    A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has beendeveloped and validated. The expanded uncertainty of solar irradiance measurements with this referencespectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelengthand the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visitsto two European UV monitoring sites as well as by regular comparisons with the reference spectroradi-ometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectralsolar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements inEurope through the Development of a Transportable Unit (QASUME) spectroradiometer and these threespectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreementis of the order of 4% over the wavelength range of 300400 nm. 2005 Optical Society of America

    OCIS codes: 120.4640, 120.3940, 120.4140, 120.5630, 120.6200, 260.7190.

    1. Introduction

    During the past decade a large number of monitoringstations have been established worldwide for moni-toring the spectrum of solar UV radiation reaching

    the Earths surface. UV monitoring is considered oneof the most important activities that have been stim-ulated in past years by the observed decreases instratospheric ozone.1,2 Moreover, the association ofsolar UV radiation with damage to human beings andto the ecosystem in general and its strong relation toatmospheric chemistry imposed the necessity of per-forming high-quality spectral UV measurements thatwould help substantially to address these issues.3Given that the UV represents only a small part of thesolar spectrum, its measurement becomes difficult,requiring high-level technology as well as sophisti-cated instrumentation and procedures.4

    The need for quality control (QC) and quality as-surance (QA) of UV measurements has been recog-nized since the beginning of the 1990s.5,6 Theestablishment of international databases of solar UVmeasurements, e.g., the European UV Database(EUVDB) established within the framework of Euro-pean Commission-funded projects and the WorldOzone and UV Database (WOUDC) hosted by theMeteorological Service of Canada, and in particular

    When this research was performed, J. Grbner ( was with the European Reference Centrefor Ultraviolet Radiation Measurements, Institute for Health andConsumer Protection, European Commission, Joint Research Cen-tre, Ispra, Italy; he is now with the Physikalisch-MeteorologischesObservatorium Davos, World Radiation Center, Dorfstrasse 33,CH-7260 Davos Dorf, Switzerland. J. Schreder is with CalibrationMeasurement Software Solutions, Kirchbichl, Austria, S. Kaza-dzis and A. F. Bais are with the Laboratory of Atmospheric Phys-ics, Aristotle University of Thessaloniki, Thessaloniki, Greece, M.Blumthaler is with the Institute for Medical Physics, University ofInnsbruck, Innsbruck, Austria, P. Grts and R. Tax are with theLaboratory of Radiation Research, National Institute of PublicHealth and the Environment, Bilthoven, The Netherlands, T. Ko-skela is with the Finnish Meteorological Institute, Ozone and UVResearch, Helsinki, Finland, G. Seckmeyer is with the Institutefor Meteorology and Climatology, University of Hannover, Han-nover, Germany, A. R. Webb is with the School of Earth Atmo-spheric and Environmental Sciences, University of Manchester,Manchester, England, and D. Rembges is with the Institute forHealth and Consumer Protection, European Commission, JointResearch Centre, Ispra, Italy.

    Received 10 January 2005; revised manuscript received 18

    March 2005; accepted 18 March 2005.0003-6935/05/255321-11$15.00/0 2005 Optical Society of America

    1 September 2005 Vol. 44, No. 25 APPLIED OPTICS 5321

  • their relationship to the users community call forstrict application of QCQA procedures to ensurethe quality and comparability of the data,7 havehelped to fill that need. QC is performed at moni-toring stations through the development and appli-cation of appropriate procedures, most of whichhave already been tested and verified through in-ternational collaborations among UV instrumentoperators. It is however, uncertain, at how many ofthe existing UV stations proper QC is maintained.Until now, QA has been achievedwith particularsuccessmainly through participation of instru-ments in intercomparison campaigns.8 As the num-ber of deployed instruments is constantly increasing,such campaigns have become impracticable; in addi-tion, there is a risk of damaging the instruments oraltering their optical characteristics during transpor-tation and the interruption of their regular recordsfor long intervals.

    The European Commission-funded project QualityAssurance of Spectral Ultraviolet Measurements inEurope through the Development of a TransportableUnit (QASUME) was launched in December 2001(Ref. 9; see http:lap.physics.auth.grqasume). Itaims at providing QA to spectral solar UV measure-ments conducted now by spectroradiometers operat-ing in Europe by establishing a reliable transportablespectroradiometer system that can be transported toany UV monitoring site in Europe and provide collo-cated measurements with the local site instrument.This on-site QA exercise should be viewed as an al-ternative to the intercomparisons performed previ-ously, in which spectroradiometers from differentparts of Europe were gathered at one location to per-mit their performance to be assessed during simul-taneous measurements.10 The advantages of theproposed approach are that local monitoring instru-ments do not need to be transported and are used intheir natural environment during the intercompari-son; furthermore, a site can be visited at regular in-tervals for checks on its stability over extended timeperiods. While this procedure offers a more realisticevaluation of a monitoring site, it places strict criteriaon the performance and operation of the travelinginstrument, which must be proved to be stable at alevel against which all other instruments will bejudged.

    The QASUME traveling unit is composed of thespectroradiometer, its calibrating unit, an angularresponse unit, and a heliumcadmium laser. The lasttwo items are provided to the local site operator fordetermining the angular response of the sites detec-tor and the slit function of the spectroradiometer,respectively. In this paper, only the reference spec-troradiometer and its associated calibrating unit arediscussed. The home site of the QASUME travelingunit is the European Reference Centre for ultravioletradiation measurements (ECUV) at the Joint Re-search Centre of the European Commission at Ispra,Italy.

    2. Instrumentation

    The spectroradiometer consists of a commerciallyavailable Bentham DM-150 double monochromatorwith a focal length of 150 mmmonochromator andwith 2400 linesmm gratings. The wavelength rangeis 250500 nm, and the entrance and exit slit widthwas chosen to yield a nearly triangular slit functionwith a full width at half-maximum resolution of0.8 nm.11 The smallest wavelength increment is0.0025 nm. The spectroradiometer has two entranceports, which can be selected by a remotely controlledinternal mirror. The solar irradiance is sampledthrough a specially designed entrance optic (CMS-Schreder, Model UV-J1002) which is connected to oneport of the spectroradiometer through a quartz fiber.The second entrance port holds a pencil ray mercurylamp (Oriel, Model 6035) which is used to check thewavelength setting of the spectroradiometer. UntilSeptember 2003 a side-window-type photomultiplier(PMT) was used as a detector; then it was replacedwith an end-window-type bialkali PMT (electrontubes 9250QB). The photocurrent is measured with asix-decade current amplifier, integrated for a 100 mstime window, digitized, and transferred to a com-puter for further data treatment and storage.

    Because the instrument is designed for outdoor so-lar measurements, the whole spectroradiometer sys-tem including the data-acquisition electronics iscontained in a temperature-controlled box that is sta-bilized to a predetermined temperature with a preci-sion of 0.5 K.

    Initially, the spectroradiometer was characterizedin the laboratory; the results pertaining to the mostimportant parameters are discussed below.

    A. Wavelength Scale

    The wavelength scale of the spectroradiometer wasinitially determined by use of spectral emission linesfrom mercury, cadmium, and zinc spectral dischargelamps. We obtained the relationship between thegrating angle and the wavelength by simultaneouslyminimizing the residuals at all measured spectrallines. The best result is obtained with a second-orderpolynomial with the resultant residuals all below0.02 nm. The stability of the wavelength scale is mon-itored with the pencil ray mercury lamp mentionedabove. Before every solar measurement, a fast scanthrough the 289.9 nm spectral line is used to checkthe wavelength alignment of the spectroradiometer.The wavelength repeatability, based on these mea-surements, is usually better than 0.01 nm during oneday of continuous measurements.12 However, the re-spective wavelength scales of the two entrance portswere found to differ between successive site visits, soa different method was required for checking thewavelength scale of the measured solar spectra. Theselected method uses a validated extraterrestrialspectrum13 in the wav


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